Rare-earth doped ceramics have shown great potential as gain materials for high energy lasers in many industrial and defense applications. However, significant deficiencies exist in current ceramic technology to meet the demands of such applications. Rare-earth doped ceramic laser materials typically suffer from several key drawbacks including; i) low doping density due to the low solubility of rare-earth elements in typical solid hosts, limiting the optical gain to a relatively low level; ii) rare-earth ion clustering due to inhomogeneous distribution of doped ions at moderate and high doping levels; iii) rare-earth to rare-earth, rare-earth defect, and rare-earth grain-boundary interactions; and iv) other nonlinear effects lead to degradation of light emission, or nonlinear saturation of optical gain. In addition to doping level limitation, doped single crystal materials are expensive and difficult to make in large enough volumes for high power applications.
In comparison, ceramic laser materials have advantages of low cost and can be fabricated into arbitrary shapes with large volume or quantity, and with higher dopant concentration. However, ceramic materials can be problematic when used in optical application due to the interaction of dopants with scattering centers such as grain boundaries and various defects that can lead to reduction of optical gain, and serious material degradation.
Rare-earth compounds, such as various rare-earth oxides and silicates are promising alternatives to doped (single crystal, glass, or ceramic) materials. Unlike doped materials, rare-earth compounds contain light emitting rare-earth ions as integral components of their periodic crystal structures, rather than extrinsic, randomly introduced dopants. Thus, the rare-earth ions are periodically distributed in the crystal compounds with extremely high density. Various methods have been used for producing rare-earth compound crystals such as wet-chemistry, sol-gel methods, metal-organic molecular beam epitaxy, and magnetron sputtering. However, research has shown that most of the materials produced using these methods are of poor crystal quality, and show weak light emission even after high temperature annealing. Moreover, producing enough of these materials to create a large enough volume of gain materials for commercial waveguide or laser structures has proved challenging. Because annealing becomes less effective when thick materials are grown, these materials are typically produced with thicknesses of the order of hundreds of nanometers.
Single crystal rare-earth compounds such as single crystal erbium chloride silicate and its alloy with yttrium chloride silicate have recently been produced in nanorod form. Using these methods, single crystal erbium chloride silicate and yttrium chloride silicate compounds with high and controllable erbium (Er) density have been synthesized. Moreover, highly crystalline single crystal erbium chloride silicate and yttrium chloride silicate compounds have been shown to lead to strong light emission around 1.53 μm wavelengths, and have been shown to have superior optical properties including strong light emission, weak up-conversion, larger signal enhancement, and higher optical gain when compared with rare-earth doped materials or other polycrystalline rare-earth compounds. However, although single crystal rare-earth compound nanorod materials possess many superior optical properties, their size and morphology make them poorly suited for applications such as in high energy laser materials.
Accordingly, there is a need to develop nanorod-based materials in large volume with various required shapes for future high energy lasers used in defense and industrial applications. In particular, there is a need to develop large volumes of high-quality rare-earth compositions that retain the key advantages provided by ceramic materials (such as low cost and large volume or quantity), while providing along the superior optical properties of single-crystal materials.